I dont understand: when I had swaped 'Impossible' to '0' my program solved more tests than late.

there is no test where the answer is impossible, you can can make every mark you want

Even below 1.0 or above 10.0?

No: "1 ≤ X, Y ≤ 10"

I also do not see under what conditions it can be "impossible". Even if y = 1.0, then this is the result of rounding, but the ratio of the sum of estimates to their number, equal to 1.0499, really approaches.